1. Field
An embodiment of the invention relates to an illumination optical system, an exposure apparatus, and a device manufacturing method. More particularly, the embodiment relates to an illumination optical system suitably applicable to an exposure apparatus for manufacturing devices, for example, such as semiconductor devices, imaging devices, liquid crystal display devices, and thin film magnetic heads by the lithography process.
2. Description of the Related Art
In the typical exposure apparatus of this type, light emitted from a light source travels through a fly's eye lens as an optical integrator to form a secondary light source as a substantial surface illuminant consisting of a large number of light sources (generally, a predetermined light intensity distribution on an illumination pupil). The light intensity distribution on the illumination pupil will be referred to hereinafter as “pupil intensity distribution.” The illumination pupil is defined as a position where an illumination target surface becomes a Fourier transform plane of the illumination pupil by action of an optical system between the illumination pupil and the illumination target surface (a mask or a wafer in the case of the exposure apparatus).
Light from the secondary light source is condensed by a condenser optical system and thereafter illuminates the mask with a predetermined pattern thereon in a superimposed manner. Light passing through the mask travels through a projection optical system to be focused on the wafer, whereby the mask pattern is projected and exposed (or transferred) onto the wafer. The pattern formed on the mask is highly microscopic and, in order to accurately transfer this microscopic pattern onto the wafer, it is essential to obtain a uniform illuminance distribution on the wafer.
In recent years, for realizing an illumination condition suitable for faithful transfer of the microscopic pattern in any direction, there is the proposed technology of forming the annular secondary light source (annular pupil intensity distribution) on the illumination pupil at or near the rear focal plane of the fly's eye lens and setting the beam passing through the annular secondary light source, in a linearly polarized state having the direction of polarization along a circumferential direction thereof (hereinafter referred to simply as “circumferentially polarized state”) (e.g., cf. International Publication WO2005/076045)